When the carrier frequency of the radio communication is relatively low, for example 400 or 500 MHz, the dimensions of the casing of the apparatus are too small for it to be possible to use a so-called quarter-wave monopole or dipole antenna, that is to say an antenna whose conductive strands have a dimension of the order of a quarter of the wavelength. For a frequency of 400 MHz (wavelength of 75 centimeters), the length of the strand would be of the order of 20 centimeters and strands of this length cannot be housed in a casing with sides of 8 or 10 centimeters. In particular, it is not possible to use a monopole antenna configuration in the form of a conductive strand arranged at right angles to a ground plane.
It is necessary to use more sophisticated antenna designs, slotted or meandering, and the ground plane generally consists of all the electronic elements inside the casing (printed circuit boards, display screen, power supply batteries). The antenna itself is necessarily very close to this ground plane and it is essential to be able to place it inside the casing in such a way that it has a sufficient efficiency of illumination despite this proximity.
More often than not, the main faces of the casing that is assumed parallelepipedal will be occupied largely by a display screen considered to form part of the ground plane. These main faces are, for all practical purposes, not available to place an antenna thereon or even a significant antenna portion. It is known to those skilled in the art that the metal frame and/or the main printed circuit board (PCB) on which the different constituent elements of the apparatus are placed form an integral part of the antenna. Nevertheless, one way of exciting the antenna in the casing then consists in placing most of the conductive surfaces of the antenna driver unit on a small side of the parallelepipedal casing, sufficiently far from the electrical elements which constitute the ground plane.
The electrical field lines leave from the small side to what is considered to be the ground plane.
The efficiency of illumination of the antenna, which is equal to the ratio of the actual radiated power to the electrical power accepted by the antenna, is degraded on the one hand by a poor distribution of the radiated electrical fields and on the other hand by the presence of the casing which is passed through by these field lines and which causes dielectric losses; the casing is often made of ABS (acrylonitrile-butadiene-styrene), which is a lossy material.
In this context, the aim of the invention is to propose a novel antenna configuration in a casing of an electronic apparatus, which makes it possible to obtain the best possible efficiency of illumination.